The use of capacitors for reactive power compensation is widely known in the prior art. The main reason for the reactive power compensation is the more efficient use of the electric lines for transfer of electric power. It may thus be installed for network losses reduction, for over-load relief and for starting of large motors. Since the reactive load is constantly varying during day and night the reactive power compensation apparatus must be capable of connecting a selectable capacitive reactance to the line. In order to make this possible a plurality of capacitor units are assembled in a capacitor bank where the capacitor units are organized in modules which are selectable connected or disconnected to the line.
The capacitor itself comprises two electrodes separated by a dielectric medium. A capacitor for use in high voltage comprises a plurality of packages of wounded electrodes and dielectrcicum. The packages are connected with each other and stacked alongside in a container filled with an insulating liquid. The capacitor contains internal fuses and pressure relief valves. The enclosure, which is a metal construction, is expandable to allow the insulating oil to expand by temperature. Sometimes this fluctuating expansion creates a fatigue breakdown of the container and the insulating liquid will leak out. The insulating oil is however flammable, especially at high temperatures Thus in order to provide sufficient cooling and to prevent the spread of fire the capacitor bank must be placed in the open.
The switching means is either a mechanical switch or a semiconducting switch. A mechanical switch comprises a pair of contact parts relatively moveable to each other and an actuator for moving the contact parts. A semiconducting switch comprises a semiconducting element that by an electric control signal resumes an open or a closed circuit. The choice is very much depending on the voltage range of operation. Thus for the low voltage (LV) range the choice of switching device is trivial. Either a mechanical switch such as a relay contactor or a semiconducting switch may be chosen depending on performance and cost.
From U.S. Pat. No. 5,420,495 a transmission line power flow controller is previously known. The controller includes a capacitor having a variable capacitive impedance selectively inserted in series with the line. This variable impedance is achieved by connecting and disconnecting a plurality of capacitor units. As a switching means a thyristor switch is used which is sufficient rapid and having a high rate of switching operations without failure?
The choice of a switching device in the MV and HV region is much more difficult. The semiconducting switches get more expensive by voltage and arrangements must be made for cooling. For MV and especially HV the semiconducting switch must contain a plurality of series connected elements that needs an extensive control equipment. The mechanical switch must be designed with a greater contacting surface and the contact parts must in the open position be separated by a greater distance. This result in a heavier construction and the actuator must be constructed to rapidly move the contact parts in a controlled way. There is also the problem with arcs arising between the contact parts, which arcs must be extinguished to achieve an open circuit. The arc damages the contact surface and often there must be a first set of contact parts for closing and opening the circuit and a second set of contact parts for handling the arc.
One such known mechanical switch is a circuit breaker. A circuit breaker is capable of connecting and interrupting a short-circuit current. Thus a circuit breaker is a heavy construction where the contact parts are contained in a breaking chamber filled with a protective gas and an actuator comprising a spring package for effecting the movement. The circuit breaker is designed especially for interrupting a short-circuit current. In a first sequence of the opening operation the contact parts are separated by the actuator and an electric arc will arise between the contact parts. In a second sequence of the opening operation the arc will be extinguished by a blast of the protective gas at a current zero crossing. Due to high acceleration and especially retardation forces the circuit breaker has a low rate of reliable switching operations before service is needed.
Another switching means is known as a mechanical switch or often known as just a switch. A switch is capable of connecting and interrupting a load current only. Normally a load current is much lower than a short-circuit current. Thus a switch must be protected by an upstream circuit breaker. A switch is a less heavy construction than a circuit breaker. It also comprises an actuator and a contact configuration which must be able to separate rapidly and to achieve an open contact distance that makes the switch free of re-striking. The acceleration and deceleration forces are lesser than the circuit breaker.
Known mechanical switching means are delicate in construction and provide a low rate of switching operations. This affects the way of controlling the reactive power compensation. Thus in order not to stress the switching means the capacitor bank is organized in fewer modules and the switching is restricted to a few occasions per day. Within the MV range there is also known a contactor for rapid switching of motors, furnaces and capacitor banks. The connecting equipment of a contactor comprises a pair of contact parts exposed in a vacuum chamber or a SF6 chamber. The contacts of a vacuum contactor are normally operated by a solenoid actuator. This reduces the number of mechanical parts involved in the operation maneuver and thus the vacuum contactor has a higher rate of operation before service is needed. However, the solenoid actuator comprising a rod of magnetizable material that is drafted into an electro magnet has a fair acceleration phase but a very abrupt retardation phase. The abrupt deceleration affects the long term performance of the contact parts. Thus a vacuum breaker normally has a short duration before maintenance is required.
Switching a capacitor bank may cause switching transients. Such transients may damage not only the switching means but also the capacitor means and cause tripping of a frequency converter overvoltage protection. The energizing switching of a capacitor is regarded at the time of connection as a short circuit. If then the potential on either side of the switching device in unequal there will be a sudden rush of current. Thereby the capacitor is generating a switching transient that comprises a current component and a voltage component. The rush-in current surge is of high amplitude and frequency and has a very short duration. Transients causing high over voltages could also occur when disconnecting a capacitor due to re-strikes in the switch/circuit breaker.
The actuator of a mechanical switch comprises a spring construction which is released to apply a moving force on one of the contact parts. By the release of the spring the contact part will continuously accelerate from a first position until it reaches a second position where the moveable contact part suddenly stops. The retardation force is severe and a potential damage of the breaker is obvious. This will affect the rate of switching operations before service is needed. Once the spring is released the movement of the contact cannot be controlled. In comparison to a projectile launched from a cannon the action could be described as ballistic. Thus with a ballistic actuator the movable contact part will be hurled away in a ballistic manner. Once the force has been released the movement of the moveable contact part cannot be controlled.
It is also known a magnetic actuator such as a solenoid. By this actuator an electromagnetic force is applied to the moveable contact part.
The force is applied the moveable part at the beginning of the movement and then decreases by the movement. The movement of the moveable contact part may be reckoned as a ballistic movement. By this is meant that the force which will move the contact part from the first position to the second position is applied at the very beginning of the movement.
With a ballistic actuator there is no possibility to alter the speed of the contact part once the movement has started. The movement of the moveable contact part has to be estimated in advance. Thus the spring force or the magnetic force has to be released in advance in order for the switch to assume an open or closed status some moments later. However the contact speed may be affected by the latching mechanism, the spring condition, the magnetic force achieved and by the current strength to handle by the contact part. Thus each time the actuator is performing a closing or an opening operation the time period for the contact part to reach its second position is varying in an uncontrolled way.